Cashback way too small

I am on Discovery Vitality and I am told that I qualify for 25% cashback when I buy vitality qualifying products at Woolworths so I have been buying a large majority of my fruit, veggies and other healthy goods at Woolworths and spend large amounts but when it's time to receive my cashback they say I only qualified for R40 or R50. So I'm not sure if there's a problem with my account or I misunderstood the cashback concept or what but I've decided to just cancel my Vitality because it doesn't seem to be value for money at all.